1. Field of Invention
This invention pertains to digital radio frequency (RF) links, particularly digital RF links used on the modern battlefield.
2. Purpose of the Invention
This invention identifies a process for determining the performance of radio frequency (RF) links in the Army's Enhanced Position Location Reporting System (EPLRS) with a high level of statistical confidence. EPLRS is an integrated communications system that provides near real-time data communications, position/location, navigation, identification and reporting information on the modem battlefield. The system, which may include 300-1500 terminals in a division with up to five NCS's, utilizes spread spectrum technology and frequency hopping error detection and correction and is capable of supporting multiple communications channel operations. Understanding RF link performance is critical to effective use of EPLRS. RF link performance is broadly characterized by the ratio of desired EPLRS received signal to noise within the communication channel (the S/N ratio). The present EPLRS capacity model simulations use various parameters to characterize the desired signals, propagation characteristics and communication and battlefield scenarios. While the signal models work quite well under a number of scenarios, the nature of the problem is such that the models are not able to account for many situations that can and do occur on the battlefield. The present invention uses statistical methods to account for the variability associated with certain parameters used to compute the EPLRS RF link performance. For example, when a building is constructed, engineers design it with a certain amount of margin based on statistics such as steel beam tensile strength and wind conditions which stress these steel structures on any given day. In order to avoid over design and under design, statistics may be used as a tool to provide margins to achieve a quantitative level of confidence. In the case of a communications system such as EPLRS, the RF noise background and RF propagation loss are two parameters that are hard to model with great accuracy. Statistical methods can provide a quantitative level of confidence, based on measured data and certain numerical assumptions.
The key parameter used to measure a single EPLRS RF link performance is the Probability of Communication (PCOM) which is a function of S/N. PCOM is the probability of a single EPLRS pulse being successfully received over a single link in one direction. The EPLRS system employs four modes of operation which provide various levels of anti-jam capability. These algorithms provide a Probability of Communication (PCOM) for each EPLRS RF link based on mode of operation and random Gaussian (white) noise in the RF noise environment.
The method of this invention can be used to generate an input file required by Communication Electronics Command (CECOM) EPLRS Capacity Model version 6.2. This tool can also be used to analyze all possible permutations of RF links supporting an EPLRS network with and without broadband (gaussian noise) jamming.
The EPLRS PCOM analysis tool can provide a user with a quantitative level of confidence for predicting EPLRS RF link performance. The Army currently uses a model called the "Terrain-Integrated Rough-Model" (TIREM) to calculate a mean RF propagation loss value given a terrain profile along the RF link path. For background, see DOD Electromagnetic Compatibility Analysis Center (ECAC) document ECAC-HDBK-93-076, titled "TIREM/SEM Handbook" dated March, 1994, chapter 5 "Model Limitations," page 5-5 incorporated herein by reference as if fully set forth. According to the Handbook, TIREM's mean calculated RF propagation loss is -0.6 dB with a standard deviation of 10.5 dB. The TIREM model is static, however, and does not adequately account for changes in RF signal and noise conditions, resulting in miscalculation of RF link propagation losses.
While one could attempt to refine the model in order to account for various propagation conditions, the possible combinations are simply too numerous. Rather than attempting to improve the model, the present invention performs a statistical analysis of all the model parameters required to calculate the EPLRS S/N ratio, then identifies the parameters that have the most inaccuracies, making those parameters the limiting factor during subsequent calculations. For example, the present invention accounts for the inaccuracy contained in the TIREM mean value, by using a process that provides a margin based on random variables and the normal distribution function. This margin provides a stressing factor to the mean calculation of S/N which allows for a higher confidence that the model will represent the real world by a certain percentage of time. Another example is the RF noise background used for the noise calculation, which is separate from the broadband jamming noise. Reference is made to ITT textbook entitled "Reference Data For Radio Engineers" fifth edition (October, 1968) page 27-2 FIG. 1 "Median Values of Average Noise Power from Various Sources," incorporated herein by reference as if fully set forth. These values are assumed values that can be used as stress factors when calculating the N in the S/N ratio.